Image courtesy USDA/ARS.The enormous scale of agricultural production—and
the resulting environmental impacts—has made developing
more sustainable and environmentally friendly
agriculture a significant 21st-century challenge.
Stretching Limited
Resources: Plants That Can
Hold Their Water
Water is one of the most precious resources
on earth. Although it covers 71
percent of the planet, only a small fraction
is available as freshwater for use by
humans, animals, and plants. Competition
for water resources is increasing as
human populations expand and water
demand grows. Increasing temperatures
caused by climate change are likely to
mean thirstier plants and people, putting
even more pressure on limited water
supplies. Although some crops have been
bred for drought tolerance, the genome
sciences have vastly enhanced the ability
to manipulate this important quality in
many different species.
The Resurrection Plant has
attracted the attention of scientists interested
in deciphering how it survives
extreme drought conditions. Seeds, also,
may provide clues to plant mechanisms
for surviving on limited water; many plant
seeds go through periods of intense dryness
before germinating. Identifying the
genes and mechanisms that allow seeds
and drought-resistant plants to stay alive
could help scientists create more droughtresistant
crops for the future.
Image courtesy USDA/ARS.One approach to developing droughtresistant
plants is to identify the genes
behind the physical mechanisms through
which certain species manage to survive
drought. Another approach is to decipher
the signaling system through which
normal crop plants activate such genes
under conditions of extreme stress, and
find ways to trigger those signaling activities
more quickly. Other approaches
focus on "predrought preparation" by
encouraging certain growth patterns or
behaviors that would help plants survive
drought, should it occur.
Reducing Fertilizer Use
Historically, farmers in developed countries
have blanketed their fields with enormous
quantities of fertilizers several times per
year to ensure maximal plant growth. But
a rising awareness of the negative consequences
of this practice—downstream algal
blooms that block sunlight, deplete water
of oxygen, and kill marine and aquatic
organisms—has prompted researchers to
take a closer look at how to effectively fertilize
crops without jeopardizing the health
of downstream ecosystems.
Genomic sciences are helping scientists
to pinpoint exactly when and how
plants actually use nutrients so they can
advise farmers on the most effective
times to apply fertilizers. Scientists are
also gaining insights on genes that help
plants to efficiently extract nutrients
from soil; plants that can utilize existing
nutrients more fully would also reduce
the need for fertilizers.
Future Directions:
What if Plants Could
Clean Up Pollution?
Contamination by harmful metals or chemicals
can cause vast swaths of land to become
unusable. In some cases, no plants
will grow in contaminated soil; in others,
plants will grow but can pass harmful contaminants
up the food chain to consumers.
Barren land resulting from zinc contamination from a smeltry that
operated here from 1890 to 1980. Image courtesy USDA.Historically, contaminated soils are either
avoided or treated by scooping topsoil
away to landfills—a measure that is
costly, wasteful, and disturbing to natural
systems. But new alternatives have recently
surfaced: Scientists are discovering some
amazing plants that can actually clean up
soil contaminants themselves, through a
process known as
phytoremediation.
Arabidopsis halleri thrives even in
soils with astoundingly high concentrations
of typically harmful metals
such as zinc and cadmium. Where
most plants would be poisoned by an
accumulation of 1,000 parts per million
(ppm) zinc or 50 ppm cadmium in
their shoots, this plant can withstand
as much as 21,500 ppm zinc and 350
ppm cadmium with few or no symptoms
of toxicity.
Arabidopsis halleri and
other plants, when grown on contaminated
soil and then appropriately
disposed of, can be an effective tool
for cleaning up contamination.
Another plant,
Amaranthus retroflexus,
has been shown to effectively
remove cesium (the radioactive form
of which is present in the environment
as a byproduct of above ground
nuclear testing) from soil. Researchers
estimate that two to three yearly
crops of the plant could clean up an entire
contaminated site in less than 15 years.
Phytoremediation research could also
help to identify plants that can survive in
acid soils—those with naturally occurring
high levels of aluminum. Acid soils have historically
been avoided because they limit
crop productivity, but they are widespread,
comprising over half of the world's 8 billion
acres of land that would otherwise be
considered arable. Scientists are working
to identify the genes that help some plants
deal with high aluminum concentrations,
with the ultimate goal of developing more
tolerant crops that farmers could cultivate
on lands currently considered marginal.
New Approaches:
The Science of Metagenomics
No plant is an island. In fact, plants are surrounded
by millions of microorganisms that play a crucial
role in their survival. Microorganisms manufacture
nutrients, for example, by converting atmospheric
nitrogen into ammonia and recycling nutrients from
decaying plants and animals. Some microorganisms
living in soil actually protect plants from diseases—when these microorganisms are removed, the
plants are far more susceptible to infection.
The new science of metagenomics bypasses
the need to isolate and culture individual species,
enabling scientists to apply genomic analysis to
 entire microbial communities in the environment
at once. Metagenomics offers scientists unprecedented
access to crucial soil microorganisms and
can reveal much about a microbial community's
members and the functions they are performing. For information on metagenomics and plant-associated microbes, see the National Research Council report, The New Science of Metagenomics: Revealing the Secrets of Our Microbial Planet.
A better understanding of microbial communities
in and around plants could lead to ways
to harness the power of these communities to
produce healthier and more robust crops. One
example of this is an approach called "no-till"
farming. In no-till farming, the plant biomass that
remains on a field after a crop is harvested is
simply left on the soil surface rather than being
plowed under before reseeding. This leaves soil
microbial communities intact and allows them
to continue to perform their vital functions.
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Fighting Plant Pathogens
in a Changing Climate
In 1989, a half-inch flying insect known as
the glassy-winged sharpshooter hitchhiked
from its home in the southeastern United
States to southern California. There, it
found a hospitable climate and a bountiful
supply of what quickly became its favorite
food—grape vines. Within two years, the
insect had made a name for itself as one of
the most serious threats ever to face the
California wine industry.
Glassy-winged Sharpshooter. Image
courtesy ARS/USDA; photo by
Peggy Greb.The glassy-winged sharpshooter is a
voracious eater, but that isn't how it shut
down most of the vineyards in California's
Temecula Valley and continues to threaten
vineyards and other crops elsewhere. The
true culprit is another hitchhiker—a bacterium
called
Xylella fastidiosa. Glassy-winged
sharpshooters inadvertently carry
Xylella in
their mouths as they flit from plant to plant,
injecting the bacterium into healthy plants after
feeding on sick ones. The resulting
infection is known as Pierce's disease,
which causes the vines to slowly die
over a period of one to three years.
As global climate change brings
warmer temperatures, biologists
predict that the ranges of
Xylella and
other crop pathogens could expand.
Areas that are currently too cold for
the sharpshooter may eventually become
more hospitable, causing the
threat of Pierce's disease and others
to continue to grow.
Currently, there are few ways to
fight Pierce's disease. Farmers prevent
its spread by removing infected
crops at the first sign of symptoms,
and they also use insecticides to
contain its insect vector, the glassywinged
sharpshooter. But there is
new hope that grapes may be able to
resist Pierce's disease on their own.
Some plants appear to be more susceptible
to the disease than others; scientists
are hard at work in search of the genes
that allow certain plants to resist infection.
Uncovering genes for
Xylella resistance
could help breeders grow plants with natural
immunity to Pierce's disease—reigning
in this fierce pathogen even as climate
change expands its potential reach.
This web page is based on the National Academies' educational booklet
New Horizons in Plant Sciences.
